The invention relates to disconnect switches for use with the start or auxiliary winding of a single phase AC induction motor
A single phase AC induction motor typically includes a squirrel cage rotor and two stator windings--a main winding and a start or auxiliary winding. The main winding is connected across an AC supply line and normally has low resistance and high inductance. The start or auxiliary winding is physically displaced from the main winding and has high resistance and low inductance. In a split phase type AC motor, the physical displacement of the motor stator winding together with the electrical phase displacement produced by the different resistance and inductance values generates a rotating field sufficient to provide a low level starting torque. After the motor has accelerated to 75-80% of its synchronous speed (cut-out speed), a disconnect switch opens and the start winding is disconnected from the AC power source. The disconnect switch is typically a mechanical centrifugally actuated type switch.
Other types of single phase AC induction motors for higher torque applications include capacitor-start type motors and capacitor-start-run type motors. In these types of motors, a capacitor is connected in series with the auxiliary winding to provide higher starting torque. As with the above noted split phase type motor, the auxiliary starting circuit is opened by a disconnect switch after the motor has accelerated to approximately 75-80% of its synchronous speed (cut-out speed).
The present invention arose during continuing development efforts connected with pending U.S. Pat. application Ser. Nos. 06/680,488 and 06/680,489, filed Dec. 11, 1984, regarding efforts directed to replace the mechanical centrifugal disconnect switch assembly for the auxiliary winding. In FIG. 1, main winding 1 and auxiliary winding 2 of an AC motor are connectable to an AC power source 3. The motor circuit may be a split phase type, or a capacitor-start type as noted above. When the motor reaches a given threshold or cut-out speed, switch 4 is opened to disconnect auxiliary winding 2 from AC power source 3.
Various types of switches 4, and controls therefore, are known. In one arrangement, a centrifugal actuator is mounted on the shaft of the motor rotor. At a given threshold or cut-out speed, centrifugal weights are displaced radially outwardly to open the switch. While this type of actuation has proven useful for its intended purpose, it is nonetheless subject to the problems inherent in any mechanical type actuation system, including limited life, fatigue, friction, vibration, mounting position, contact wear, and so on. Also, the mechanical centrifugal switch has a radial extent and blocks axial air flow through the motor which may impair cooling. Furthermore, the mechanical centrifugal switch has a given axial extent, thus requiring extra axial room in the motor, which may be objectionable in applications where motor length is critical.
In another known start or auxiliary winding disconnect system, Hall effect sensors or pick-up coils are used to detect motor shaft RPM to actuate a start or auxiliary winding disconnect switch. This approach may be objectionable because of the requirement of adding an extra element such as a magnet on the motor shaft, and an associated pick-up coil to sense speed. These extra shaft mounted parts add to the motor length, and the associated assembly costs may be objectionable.
Another known disconnect approach is to sense total current through the main and auxiliary winding and then actuate the disconnect switch at a designated current magnitude. This requires a current sensor such as 5, FIG. 1. This sensor is in series with the main winding and the start or auxiliary winding, and is objectionable to many manufacturers because of the cost of matching the extra components to specific motor current ratings and the assembly cost of modifying the circuit and inserting such components in series in the circuit. This approach may also be objectionable due to the extra wattage and heat because current continues to flow through sensor 5 in the run mode while the motor is running, after starting.
In another disconnect system, a timer is started at initial energization of the motor. When the timer times out after a predetermined set timing interval, the disconnect switch is actuated to disconnect the auxiliary winding, i.e., switch 4 is turned off to disconnect winding 2 from AC source 3. This approach disconnects the auxiliary winding after the preselected time regardless of motor speed, and hence is limited to dedicated applications where the load on the motor is known beforehand, and the delay time set accordingly. If the motor speed is not up to a desired cut-out speed when the auxiliary winding is disconnected, the motor will stall, and will not restart even when the load is removed. Instead, the motor must be restarted.
The present invention addresses and solves the above noted and other problems in a particularly simple and cost effective manner. An electronic control system is provided for the auxiliary winding disconnect switch and provides automatic restarting cut-in or bumping of the motor speed if speed is not up to a desired cut-out speed when the auxiliary winding is disconnected. The auxiliary winding is automatically reconnected to provide additional starting torque, and if the motor still does not gain desired speed, the auxiliary winding is reconnected again, as needed, all without external intervention. Automatic multiple bumping of the motor speed during starting has been found particularly desirable in many applications.